When temperature needs to be measured to an accuracy of millidegrees Celcius or better, the use of Platinum resistance thermometers is preferred. Platinum resistance thermometers are used as primary standards to calibrate other types of thermometers. The electrical resistivity of Platinum has been extensively studied and documented in handbooks and other publications on thermometry.
For use in thermometers, It is desirable to have platinum resistance temperature sensors having resistance in the range of about ten ohms to several thousand ohms because resistances in this range are easier to measure than much higher or lower resistances. In the past, thin platinum wires have been used extensively as temperature sensors. Platinum wires that are thin enough to give easy-to-measure resistances turn out to be expensive to manufacture and fragile to handle. More recently, resistance temperature sensors have been made with thin films of platinum or ceramic substrates. The platinum film is often fabricated in a serpentine pattern to maximize resistance, and when supported by the substrate, is rugged enough for handling.
In most applications, the platinum resistance temperature sensor needs to be placed in a hermetically sealed housing for protection against contamination. The housing, with the enclosed temperature sensor and necessary connecting wires, then function as a resistance thermometer probe (hereinafter sometimes also referred to as xe2x80x9cprobexe2x80x9d). At the tip of the probe is the temperature sensor. In most applications, the probe also has an elongated stem so the temperature sensing tip can be inserted into otherwise inaccessable positions for making measurements.
There have been many prior inventions concerned with the fabrication of thin film resistor temperature sensing devices. Prefabricated thin film platinum resistor temperature sensing devices on ceramic substrates can be purchased at reasonable cost from suppliers such as Omega Engineering (some for less than $20). But ready-to-use probes, with a platinum resistor temperature sensor installed and sealed in a protective housing, may cost thousands of dollars.
It is therefore highly desirable to have low cost platinum resistor thermometer probes to be used with ohmmeters for measuring temperatures.
One embodiment of the invention provides a resistance thermometer probe comprising a temperature sensor and a housing around the temperature sensor. The housing having a temperature sensing tip for placement of the temperature sensor and a stem to enable the temperature sensing tip to reach into otherwise inaccessable positions. The stem being generally cylindrical in shape and the temperature sensing tip being an integral extension of the stem but is reduced in both diameter and wall thickness to minimize thermal capacity and to maximize heat conduction. The probe also has electrically conductive wire leads to connect the temperature sensor to an ohmmeter. Fine granular glass particles and molecular sieve are placed in the empty space left after the temperature sensor and conductive wires have been inserted into the housing. The particles provide mechanical support to the temperature sensor, and reduces convection. The use of molecular sieves enables the temperature sensor to be cleared of contaminants and have a partial vacuum generated within the probe without having to use an external pump. This is accomplished by heating the probe (before it has been sealed) to a high temperature, then part of the probe is allowed to cooled to an intermediate temperature while the probe is sealed; when the molecular sieve is finally allowed to cool to room temperature, it becomes activated to absorb substantially all the moisture and organic contaminants inside the probe leaving the temperature sensor in a partial vacuum.